List View Model

// copy and paste into play ground
import UIKit

protocol ListViewModel {
  associatedtype T:Comparable
  var items: [T] { get set }
  
  
  mutating func clear()
  func item(forIndex index: NSIndexPath) -> T
  func indexOfItem(forId id: Int) -> Int?
}


// implementation of concrete helpers
extension ListViewModel {
  var countOfContents: Int {
    return items.count
  }
  
  var hasContent: Bool {
    return countOfContents > 0
  }
  
  //sometimes count of contents is different
  var numberOfRows:Int {
    return countOfContents
  }
}

// An example of a concrete Struct implementation

struct AViewModel : ListViewModel {

  typealias T = Int

  var items = Array(1..<5)
  var countOfContents: Int {
    return items.count
  }
  
  mutating func clear() {
    items.removeAll()
  }
  
  func item(forIndex index: NSIndexPath) -> Int {
    return items[index.row]
  }
  
  func indexOfItem(forId id: Int) -> Int? {
    // does not apply to this list
    return .none
  }
}


// An example of a concrete Class implementation

class AViewModelClass : ListViewModel {
  var hasContent: Bool {
    return true
  }
  
  typealias T = Int
  
  var items = Array(1..<5)
  var countOfContents: Int {
    return items.count
  }
  
  func clear() {
    items.removeAll()
  }
  
  func item(forIndex index: NSIndexPath) -> Int {
    return items[index.row]
  }
  
  func indexOfItem(forId id: Int) -> Int? {
    // does not apply to this list
    return .none
  }
}

var strct =  AViewModel()

print("Struct View model begins with having contents \(strct.hasContent)")
strct.clear()
print("Struct View model after clear has contents \(strct.hasContent)")


var clazz =  AViewModelClass()

print("Class View model begins with having contents \(clazz.hasContent)")
clazz.clear()
print("Class View model after clear has contents \(clazz.hasContent)")